For the convenience of description, reference is now made to the attached FIGS. 9 and 10. As typically shown in FIG. 10, a prior art bicycle speed change assembly a (a rear speed change assembly) has a chain guide d which rotatably supports a guide pulley b and a tension pulley c. Via a shift link mechanism such as a parallelogram pantograph mechanism g, this chain guide d is supported on a rear end plate e at an end portion of a bicycle frame or on a bracket attached thereto, pivoted by an appropriate shaft while being urged in a direction to tension a chain C.
The pantograph mechanism g comprises a link base h supported by the bracket f, an inner and an outer links i and j which are pivotally connected to the base end of a link base h, and a movable member k. The inner and the outer links i and j extend forward. The movable member k is the place where the chain guide d is rotatably supported.
The chain guide d is elastically urged by a coil spring (not shown in the FIGS.) in the direction to urge the chain C, i.e. in the clockwise direction in FIG. 10. This is to eliminate slack in the chain C and to give an appropriate tension thereto.
With the above-mentioned arrangement, when the speed change cable T, which is connected to the pantograph mechanism g, is pulled or released with the use of speed change operation lever (not shown), the pantograph mechanism g deforms to cause the chain guide d, which is supported by the movable member k, to move inward axially of a hub shaft n, forcing the chain C to move over to a desired sprocket m of the sprocket cluster M.
In this type of speed change assembly a, it is desirable for the sake of improved speed changing performance that the teeth areas of the guide pulley b and each sprocket m on the sprocket cluster M should be within an appropriate proximity because the guide pulley b is the member responsible for moving a portion of the chain onto a target sprocket before it is engaged with the teeth of a present sprocket. If the guide pulley and the clustered sprockets are located too far away from each other, the guide pulley will have to travel over a long distance to complete a chain shift operation, resulting in a poor speed change response.
For this purpose, it is desirable to design the speed change assembly in such a way that the chain guide d would not travel in parallel to the hub shaft n but to travel along a line practically parallel to a line defined by connecting each edge of sprocket m of the cluster M, or to travel in a manner that the distance between each sprocket m and the guide pulley b would be substantially constant. The sprocket cluster M usually has a structure wherein a sprocket of a greater diameter is placed closer to the hub shaft. For the guide pulley b, this means that as it moves axially inward of the hub, it must move away from the hub shaft n.
As shown in FIG. 10, this is achieved in prior art typically by either; structuring the pantograph mechanism g by supporting the link base h below the hub shaft n and making the inner and the outer links i and j extend forwardly from this link base h and thereby slanting the pivotal shaft p and q of the inner and outer links i and j by an appropriate angle off the vertical, or otherwise, by rotatably supporting the link base h against the bracket f. By so doing an arrangement was made so that the guide pulley b would travel rearwardly downward, as shown in FIG. 9, as it moves axially inward of the hub shaft, while it maintains a substantially constant distance between itself and each corresponding sprocket m.
A problem with this type of prior art arrangement, however, is that when the pantograph mechanism g deforms, the guide pulley b supported by the chain guide d approaches the vertical V which runs through the hub shaft, as shown in FIG. 9. This makes the guide pulley b's travel l away from the sprocket M significantly smaller than the actual travel L of the guide pulley b. As a result, it is necessary in prior art arrangement that as the sprocket's gear ratio increases, a greater slant angle has to be provided for the pivotal shafts p and q, resulting in a greater amount of deformation necessary for the pantograph g, and longer pull for the speed change operation cable T. All of these pose a problem of increased overall size of the speed change assembly and the speed change lever.
Another problem with the prior art speed change assembly is that the great deformation of the pantograph link mechanism g and the resulting length of the pull of cable T pose an inherant techinical hurdle against improved operatability of the speed change operation.
Further, since the guide pulley b has to travel over a long distance, the slack in the chain C has to be correspondingly great, resulting in a long and heavy weight of the chain C, putting a limit to the weight reduction of the bicycle.
There is still another disadvantage in the prior art speed change assembly: Since the speed change mechanism a has to be mounted at an outermost location of the overall width of the bicycle, it is very susceptible to damage in case of a bicycle rollover. Especially on bicycles specifically designed for off-road cycling, popularly known as mountain bikes, the end of the speed change mechanism a is apt to catch bush or other obstacles, interfering with the speed change function.
A further problem is that since the link base h is located behind the speed change mechanism a, the speed change operation cable has to be introduced to the speed shift mechanism a from behind thereof. As a result, the cable T, originally coming from ahead, has to be curved in a U shape at the back or the side of the hub shaft, making a large loop extending toward the rear or the side of the bicycle, posing a additional safety problem that the speed change mechanism a or the loop may catch another bicycle or a pedestrian passing in proximity.
There is still another problem with this type of prior art arrangement. The speed change cable T, which usually comprises an inner cable t1 and a sheathing outer cable t2, must be laid in a U shape, and this adds a length to the cable T, as well as it increases the sliding resistance between the inner and the outer cables t1 and t2. This in turn increases the turning resistance of the speed change operation lever, decreases the operatability thereof, and in addition, calls for a great spring force for returning movement thereof, and results in a difficulty of producing smaller and lighter speed change assemblies.